Noise canceling headphones, used in combination with an audio player, provide audio streams (e.g., music, phone calls, audio books, etc.) while reducing or eliminating ambient sounds (e.g., noise) by increasing a signal-to-noise ratio via active noise cancellation (ANC).
Yet, by their nature, noise canceling headphones also prevent warning sounds within the ambient sounds from reaching the user's ear. And, because at certain times an awareness of one's surroundings is a matter of safety, it may be prudent to identify and inject warning sounds into the audio streams despite the use of noise canceling headphones.